"One Tree Hill" creator Mark Schwahn has been accused of sexual harassment by former cast and crew members of the series.

The accusations come more than five years after the show's conclusion, amid a reckoning in Hollywood during which people from across the industry are speaking up about instances of alleged sexual assault or harassment.

The allegations against Schwahn were first alluded to by a former staff writer Audrey Wauchope on social media and later detailed in an open letter signed by 18 female former cast and crew members, including actresses Sophia Bush, Hilarie Burton, Bethany Joy Lenz, Danneel Harris, and Michaela McManus.

"To use terminology that has become familiar as the systemic reality of sexual harassment and assault has come more and more to light, Mark Schwahn's behavior over the duration of the filming of 'One Tree Hill' was something of an 'open secret,'" said the letter, provided to CNN by a representative for Bush.

"One Tree Hill" ran for nine seasons from 2003-12, first on The WB and then on The CW.

"Many of us were, to varying degrees, manipulated psychologically and emotionally. More than one of us is still in treatment for post-traumatic stress," the statement continued. "Many of us were put in uncomfortable positions and had to swiftly learn to fight back, sometimes physically, because it was made clear to us that the supervisors in the room were not the protectors they were supposed to be."

The letter, first published by Variety, goes on to say that female staffers were also "spoken to in ways that ran the spectrum from deeply upsetting, to traumatizing, to downright illegal" and some "put in positions where we felt physically unsafe."

The letter alleges some were threatened with retaliation and "many of us were told, during filming, that coming forward to talk about this culture would result in our show being canceled and hundreds of lovely, qualified, hard-working, and talented people losing their jobs."

A request for comment sent to a representative for Schwahn by CNN has not been returned.

Schwahn is currently an executive producer on E!'s "The Royals," which he also created.

On Monday, a spokesperson for E! told CNN, "We are monitoring the information carefully."

"E!, Universal Cable Productions and Lionsgate Television are committed to providing a safe working environment in which everyone is treated respectfully and professionally," a statement said.

Warner Bros. Television, which produced "One Tree Hill," did not return CNN's request for comment late Monday.

On social media, the former cast and crew who signed the letter were met with support by other fellow cast mates.

Austin Nichols, who began recurring in the series in Season 6, said in a tweet, "I stand with all my OTH sisters."

Former "One Tree Hill" staffer Karin Gist, now showrunner of Fox's "Star," also wrote on Twitter, "As a woman writer who sat on that couch- I stand WITH you."

In her original social media statements, Wauchope, now a writer on CW's "Crazy Ex-Girlfriend," wrote about female staffers who would avoid sitting next to the "showrunner," now identified as Schwahn, on the staff couches so "as to not be touched."

"Sometimes we wouldn't luck out and he'd just squeeze his disgusting body in between us and put his arms around us, grinning," she wrote. "He pet hair. He massaged shoulders. I know he did more but not to me so they're not my stories to share."

In the letter written by the group of former staffers, the women said they confided in each other and "set up safe spaces to talk about his behavior and how to handle it" and warn new women.

"We understood that a lot of it was orchestrated in ways that kept it out of sight for the studio back home," the letter said, alluding to the fact that the series was shot in North Carolina. "We also understood that no one was fully unaware. The lack of action that has been routine, the turning of the other cheek, is intolerable. We collectively want to echo the calls of women everywhere that vehemently demand change, in all industries."